Methods to request and check window focus

This pull request is based on https://github.com/LaurentGomila/SFML/pull/525 by @Foaly.

It extends the `sf::Window` public interface with two methods:
* `void requestFocus()` to give the window the focus
* `bool hasFocus() const` to check whether the window is currently focused

I tested the functionality on Windows 8.1 and Ubuntu 12.04. Here's a simple test program, based on Foaly's:
```cpp
#include <SFML/Graphics.hpp>

int main()
{
	sf::RenderWindow window(sf::VideoMode(200, 200), "SFML works!");
	window.setFramerateLimit(60);

	sf::Clock clock;
	while (window.isOpen())
	{
		sf::Event event;
		while (window.pollEvent(event))
		{
			if (event.type == sf::Event::Closed)
				window.close();
			if (event.type == sf::Event::KeyReleased && event.key.code == sf::Keyboard::Escape)
				window.close();
		}

		if (clock.getElapsedTime() > sf::seconds(2))
		{
			window.requestFocus();
			clock.restart();
		}

		window.setTitle(window.hasFocus() ? "Focused" : "Lost");
		window.clear(window.hasFocus() ? sf::Color::White : sf::Color::Black);
		window.display();
	}
}
```

I haven't gotten feedback concerning the X11 check as suggested by @FRex, it would be nice if somebody could have a look at commit 2df07b2eb61a9603bedb095dff73a042c4f26453.
I agree that implementing them properly is what we should do. But in the meanwhile, while there is already *existing* documentation, it should be at least mention the "workaround".

Are the iOS and Android implementations for this targeted for 2.2 as well? Because the mobile targets are stalling the 2.2 release that it's not funny anymore. ;)